1. Field of the Invention
This invention relates to a method for preparation of a functional film, particularly a multi-layer structure film which is useful for uses in electronic devices such as semiconductor devices, photosensitive devices for electrophotography, optical input sensor devices for optical image inputting device, etc.
2. Related Background Art
Devices by use of amorphous or polycrystalline deposited films of multi-layer structures such as semiconductor films, insulating films, photoconductive films, magnetic films on metal films can be expected to have physical characteristics or uses not desired on deposited films of single layer structure and therefore have been studied agressively in recent years. Particularly from the standpoint of large area devices, multi-layer structure films having two or more kinds of amorphous layers laminated are attracting interest.
For example, investigations have been made to prepare multi-layer structure films having amorphous silicon (a-Si) layer and amorphous silicon carbide (a-SiC) layer, or amorphous silicon layer and amorphous silicon germanium (a-SiGe) layer laminated alternately according to the chemical vapor deposition method (CVD method) such as the plasma CVD method or the optical CVD method, and they are contemplated to be applied for a solar battery or other devices.
In particular, formation of a deposited film according to the plasma CVD method, while there is the problem that its reaction mechanism is not well understood, is now accepted as the best method in view of productivity and characteristics of the film and has been widely applied for production of solar batteries, electrophotographic photosensitive members, etc. However, there are a large number of parameters to be considered in the formation of a deposited film (for example, substrate temperature, flow rate and flow rate ratio of the introduced gases, pressure during formation, high frequency power, electrode structure, structure of the reaction vessel, speed of evacuation, plasma generating system, etc.). By use of a combination of a large number of these parameters, the plasma may sometimes become unstable state, whereby marked deleterious influences may be frequently exerted on the deposited film being formed.
Particularly, when forming a multi-layer structure film according to the plasma CVD method, it is necessary to control discharging or gas introduction every time when a new layer is applied.
However, if the amount of the gas introduced is changed to a great extent, the pressure within the reaction space will be changed, whereby the state of discharging becomes unstable which affects the structure and the characteristics of the film. Accordingly, if a multi-layer structure is formed only by controlling the amount of the gas introduced with discharging being maintained as practiced in the prior art, it is difficult to adjust the conditions so that films with good characteristics may be constantly obtained.
On the other hand, the method in which deposited films are formed by controlling discharging, namely exchanging the gases with discharging being turned off and turned on again after the pressure has reached equilibrium, will take an extremely long time to be disadvantageously poor in productivity. Also, in plasma discharging, discharging is not generally stabilized immediately after discharging is turned on, whereby the multi-layer film obtained as an interface has bad characteristics.